The present invention is directed to a method for generating a lattice structure with a phase skip or shift on a surface of a substrate by exposing a photo-sensitive surface in an optical interference field and developing the exposed surface.
For optical communications technology, special semiconductor lasers, which also emit in a single, longitudinal oscillatory mode at a modulation frequency, are required in order to achieve high data rates on long transmission links. A fundamentally suitable type of laser is the laser with a distributed feedback which laser is referred to as a DFB-laser wherein DFB is an abbreviation for distributed feedback and wherein the feedback of the light in the laser resonator does not occur by means of two mirrors but occurs by means of reflection grating superimposed on the entire laser structure. In general, however, a DFB-laser does not oscillate in only one mode but in two modes. Single-mode light emission, however, can be induced among other things by dividing the lattice structure into two sub-gratings whose phases are shifted relative one another by half a lattice constant, for example, by one fourth of the light wavelength. Such DFB-lasers with the phase skip or shift are disclosed, for example, in an article by H. A. Haus et al "Antisymmetric Taper of Distributed Feedback Lasers", IEEE Journal of Quantum Electronics, Vol. QE-12, No. 9, Sept. 1976, pages 532-539.
Lattice structures for DFB-lasers are currently predominantly produced by holographic lithography, for example, by exposure of a photo-resist layer applied on the surface of a laser substrate of the semiconductor material by exposure to an optical interference field, then developing the layer and etching the surface covered with the developed photo-resist layer, whereby a relief-like lattice structure having a spatial frequency corresponding to the spatial frequency of the interference field will occur in the surface of the substrate. The remaining steps of the laser manufacture occur in a known manner.
The optical interference field is produced by optical superimposition of two coherent light waveguides. However, only simple grating structures without phase shifts will occur.
For generating a lattice structure with a phase shift, electron beam writers are presently utilized as disclosed by an article by K. Sekartedjo et al, "1.5 .mu.m Phase-Shifted DFB-Lasers for Single-Mode Operation" Electronics Letters, Vol. 20, No. 2, Jan. 19, 1984, pages 80-81. Another method is a holographic lithography method with combined employment of positive and negative photoresist to produce the phase shift. This method is disclosed by K. Utaka et al, ".lambda./4-Shifted InGaAsP/InP DFB Lasers by Simultaneous Holographic Exposure of Positive and Negative Photoresists", Electronics Letters, Vol. 20, No. 24, Nov. 22, 1984, pages 1008-1010. Both of these methods have disadvantages. For example, the first method requires an extremely costly electron beam writer. The second method involves a technologically complicated method which leads one to expect high rejection rates in the laser production. In both instances, the manufacture of the lattice structure is a time consuming process.